Internet portals in China - including Yahoo!'s Chinese-language site - have signed a voluntary pledge to purge their sites of content that China's communist government deems subversive, organisers of the drive say.

The Public Pledge on Self-discipline for China's Internet Industry has attracted more than 300 signatories since its launch on March 16, said a spokeswoman for the Internet Society of China, who identified herself only as Miss Sun.

Other clauses, though, seem less innocent given China's tight control over information and the government's extreme sensitivity to criticism or political challenges. Those who sign the pledge must refrain from "producing, posting or disseminating pernicious information that may jeopardise state security and disrupt social stability". The prohibition also covers information that breaks laws and spreads "superstition and obscenity". Members must remove material deemed offensive or face expulsion from the group.

Signatories also pledge to monitor the content of foreign-based websites and block those containing unspecified harmful information.

The pledge conforms closely to Chinese government policies making internet service providers responsible for content posted on the websites they host. It is a strategy to give the internet enough room to blossom while keeping operators on notice not to push the envelope politically.

China has aggressively promoted the internet for commercial purposes. As of April, China had more than 38 million internet users and nearly 280,000 websites, according to the official Xinhua news agency. Yet the ruling Communist party is determined to curtail the web's role as a forum for free discussion and a source of information not available in the entirely government-controlled media.

A special police force monitors websites and sifts email searching for messages promoting causes such as greater political openness, the banned Falun Gong spiritual movement and independence for minority regions. The websites of human rights groups and Western and Taiwanese media are frequently blocked.

Internet cafes are required to track the sites their users visit and report attempts to visit those deemed subversive. Long prison sentences have been given to people accused of reproducing or forwarding information from such sites.

"They're trying to have it both ways. It's a difficult game to play, but they seem to be doing a not inconsiderable job of it," said Jack Balkin, a Yale University law professor who studies the internet.

China has also closed thousands of unlicensed internet cafes since a fire on June 16 at a cafe in Beijing that killed 25 people.

A spokesman for Yahoo!'s China office in Beijing confirmed that the company had signed the pledge but refused to comment further. Yahoo!'s public relations agency in the US, where the company cultivates an image of freedom and anarchic creativity, responded to an email seeking comment by saying no spokesperson was available.

Other portals the society listed as having signed the pledge include the popular Chinese websites Sina.com and Sohu.com, as well as Peking and Tsinghua universities, online media and technology companies and government offices.